1. Field of the Invention
This invention relates to telephone hybrids in general and, more particularly, to optically isolated hybrids generally used in transformerless Data Access Arrangements.
2. Description of the Prior Art
For purposes here, the term Data Access Arrangement or DAA is used to refer to a transformerless hybrid and isolation means generally, although it is understood that more components may be necessary to form a complete DAA, such as a line switch and loop current control.
See U.S. Pat. No. 5,280,526 assigned to the same assignee as this invention and is included herein by reference. The circuits shown in FIGS. 2 and 3 of the above-identified patent form the hybrid and isolation functions of a Data Access Arrangement (DAA). The disclosed DAA uses conventional operational amplifiers powered from a telephone line connected between terminal 100 and ground (except amplifier 80 in FIG. 3). Added to the DAA is a voltage regulator (Zener diode 71 and resistor 70) and a passive filter arrangement (capacitor 72) to provide a regulated voltage to the amplifiers while decoupling the amplifiers from the telephone line. The filter and regulator are necessary because the amplifiers used are of conventional design, wherein the power supply current consumed by an amplifier varies with the signal amplified by the amplifier. Without the filtering, the varying power supply current can interfere and corrupt the desired signals on the telephone line.
The DAA of the aforementioned U.S. Patent includes a hybrid that couples to the two-wire telephone line. The hybrid converts the two-wire telephone line (bidirectional) path into a unilateral receive path and a unilateral transmit path. Signals arriving on the telephone line pass through the receive path over an optical isolator 44 to a modem 75 or the like. Similarly, signals from a modem 75 or the like pass through the transmit path over another optical isolator 42 and are impressed on the telephone line.
The received signal strength presented to the hybrid from a mismatched telephone line (the telephone line being terminated at the DAA with an impedance different from the match impedance) is usually very weak, typically many tens of decibels below the signal strength of the transmitted signal from the DAA to the telephone line. For the DAA to adequately send the receive signal over the optical isolator 44, a line-powered amplifier 50 amplifies the receive signal and performs part of the hybrid function. An additional line-powered amplifier (not shown but corresponding to amplifier 80 in FIG. 3 except being powered from the telephone line) further amplifies the received signal and drives an LED within the optical isolator 44. As stated above, the amplifiers used in the DAA are of conventional design, i.e., the power supply current varies with the signal being amplified. For the line-powered amplifier driving the LED in isolator 44, variations in the power supply current to the amplifier is approximately the same as the signal to the LED. Thus, without the filter and regulator, variations the power supply currents can interfere with the received signal to the point of making the received signal useless.
The foregoing also applies to the other telephone line-powered amplifiers in the DAA, where variations in power supply current consumption may interfere with the received signal.
Thus, it is desirable to provide a DAA arrangement that allows for direct powering from the telephone line without filtering or voltage regulation.
Further, it is desirable to provide a DAA arrangement with direct telephone line-powered amplifiers that do not significantly introduce undesired signals onto the telephone line.